LeetCode "Merge Two Sorted Lists"

Simply care about the boundary cases:

class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        if (l1 && !l2) return l1;
        if (!l1 && l2) return l2;
        if (!l1 && !l2) return NULL;

        ListNode *p0 = l1->val <= l2->val ? l1 : l2;
        ListNode *p1 = p0 == l1 ? l2 : l1;

        ListNode *pa = p0, *pb = p0->next;
        ListNode *pc = p1, *pd = p1->next;
        while (pa && pc)
        {
            if (!pb)
            {
                pa->next = pc;
                break;
            }
            while (pa->val <= pc->val && pc->val <= pb->val)
            {
                //    Link
                pa->next = pc;
                pc->next = pb;
                //    Move on
                pc = pd;
                if (pd) pd = pd->next;
                pa = pa->next;

                if (!pc || !pb) break;
            }
            pa = pa->next;
            if(pb) pb = pb->next;
        }
        return p0;
    }
};